Pneumatic rubber tires are composed of elements which conventionally include a tread of a rubber composition. The tread rubber is often desirably compounded (blended or mixed with compounding ingredients) to provide a tire with a relatively low rolling resistance and a tread with reasonable wear and traction.
However, where tires are intended to be used for purposes where traction (skid resistance) is a primary consideration such as, for example, may be desired for high performance tires, the tread rubber may be compounded to enhance traction where a possibility of reduced treadwear and increased tire rolling resistance may be considered to be of somewhat lesser importance.
For such purpose of emphasizing tread rubber traction, the tread may be compounded to have a relatively high ratio of resin and processing oils to rubber (e.g. a range of about 15 to about 40 weight percent in the rubber composition) which may be more conventionally expressed in terms of 100 parts by weight rubber (phr) as being, for example, in a range of about 45 to about 120 phr.
The resin may typically have a softening point (Ring and Ball) in a range of about 20.degree. C. to about 110.degree. C.
The purpose of the resin, among other purposes, is to enhance traction of the tread. While the tread rubber may normally contain such a resin, in instances where tread traction itself is to be emphasized or enhanced, the resin content in the tread rubber may be increased to the aforesaid amount.
While the traction enhancing feature(s) of the resin may be due to various factors, the softening point of the resin is normally considered important because, as the resin softens, it undergoes a phase transition and its mechanical properties change.
Indeed, it is usually considered that the resin in the rubber compound acts to soften the rubber compound and increase its hysteresis at a rubber temperature equivalent to or immediately above the resin's softening point.
Such softening point property of the resin is desirably taken advantage of in practice where, for example, as the tire is run from a stationary, resting position to vehicle driving speeds, the temperature of the tread may increase from its stationary ambient temperature (e.g. 23.degree. C.) to a temperature of 100.degree. C. or even higher at high vehicular speeds (&gt;80 mph).
Thus, in such a circumstance, a resin with a softening point of about 30.degree. C. would be expected to soften and become very hysteretic at a tread temperature of about 20.degree. C. to about 50.degree. C. and, thus, aid in tire traction at such tread temperatures. Thereafter, as the tread temperature increases to 100.degree. C., for example, the resin would be expected to be in a softened or perhaps liquid state and the tread traction would, accordingly, be expected to be affected only to a very limited degree by the aforesaid 30.degree. C. softening point resin as the rubber temperature increased and moved further above 30.degree. C.
A considerably higher softening point resin would be desirable to enhance tread traction at the higher tread temperature associated with the higher vehicular speed.
It is suggested herein that as the resin softens, the cured rubber compound containing the resin becomes hysteretic. This means the rubber compound transfers the energy generated in the tread as it rolls into heat, which in turn results in improved traction performance.
It is recognized that various resins are typically used in formulating a tire tread for various purposes, and resins are heretofore often used in somewhat larger amounts in tire treads, particularly where tread traction on the road is a primary or large consideration.
Representative examples of such resins which, as it is understood, are or may be used to aid in tire tread traction are hydrocarbon-derived synthetic resins, coumarone-indene resins, rosin, rosin derivatives and dicyclopentadiene based resins such as, for example, dicyclopentadiene/diene resins.
Such resins may typically have softening points (Ring and Ball) within the aforesaid range of about 20.degree. C. to about 110.degree. C. and even up to about 170.degree. C.
However, few if any, of such resins are normally used in rubber tire treads which have a softening point higher than about 110.degree. C.
In practice, it is desired to take advantage of the described softening point property of the resin(s) to formulate a tire tread in a manner designed to enhance the tread traction over a relatively wide range of tire tread temperatures.
In another aspect of tire tread rubber considerations, it should be pointed out that viscoelastic properties of a rubber, or a rubber blend, for tire tread applications, are important. For example, a tangent delta viscoelastic property is the ratio of the viscous contribution to the elastic contribution for a viscoelastic rubber article subjected to a cyclic deformation. The term "tangent delta" is often referred to herein as "tan. delta". Its characterization of viscoelastic properties of rubber is well known to those skilled in such art. Such property is typically represented in the form of a curve as a temperature sweep plot of tangent delta values on a y, or vertical, axis versus temperature on an x, or horizontal, axis.
Although various rubber compositions are taught to provide various benefits, some for tire treads, it is desired to provide a pneumatic tire having a rubber tread having enhanced traction qualities over a relatively wide temperature range and, thus, a tan. delta characteristic which maintains a high tan. delta as the temperature increases.
In the description of this invention, rubber compound, sulfur-cured rubber compound, rubber composition, rubber blend and compounded rubber are used somewhat interchangeable to refer to rubber which has been mixed with rubber compounding ingredients. Such terms are well known to those having skill in such art.